Aug 16, 2010

Reminder: JMS is client server infrastructure; update broker ∴ update client

Often we get the following question:
If I upgrade the broker to version 5.x, do I also need to upgrade all my clients?
 The short answer is:
maybe, but error on the safe size and upgrade your clients if it does not cause too much disruption.

For the longer answer there are at least two things to consider:
  • Does the reason for upgrade include the need for fixes that affect client side code? If yes, then obviously update all clients. (Issues of this kind typically focus on some aspect of the JMS API or consumer delivery semantics.)
  • Is there an increment to the openwire protocol version? if so, does it affect me? read on...

Does an update to the openwire protocol version affect me?
The openwire protocol is the set of commands that is used to communicate between an ActiveMQ client and an ActiveMQ broker (and from broker to broker in a cluster scenario). The openwire protocol supports version negotiation such that an old client can negotiate the lowest common version with it's peer and use that version. As a result, in most cases, old clients can work as expected with a newer broker.

There are two potential pitfalls that you should be aware of:
  • fixes/features that depend on the openwire version update.
  • the ever increasing and incomplete version testing matrix.

Fixes or features that depend on the openwire version update
These are typically fixes that require additional information to be passed from the clients to the broker or vice versa. Some examples include the addition of a last delivered sequence id parameter to a consumer close command such that the redelivery count could be more accurately calculated. Another is the addition of a reconnecting flag to a connection command that allows duplicate suppression to be implemented consistently at the transport connection level. In some cases it is not obvious if an issue requires a protocol update without some consideration of the implementation, if in doubt ask on the activemq mailing list.

The ever increasing and incomplete version testing matrix
With every protocol version change, there are new additions to the client/server testing matrix. In ActiveMQ, virtually all tests assume a uniform openwire version, with the exception of a few that validate negotiation. The net result is that validation of the compatibility matrix is largely completed by the community. This works in practice but it is important to be aware of. If you are in doubt as to whether a particular scenarios will work across a broker version mismatch, be sure; ask the computer yourself with a little test.

In summary, If you update the broker, you also need to update the clients; or at least consider it!

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